Method of separating solid particles from a slurry with wash column separators

ABSTRACT

AN APPARATUS AND METHOD FOR REMOVING SOLID PARTICLES, SUCH AS ICE CRYSTALS, FROM A MIXTURE OF SAID PARTICLES IN A LIQUID MEDIUM, SUCH AS BRINE, WHEREIN A WASH COLUMN SEPARATOR IS USED TO FORM A POROUS BED OF SAID PARTICLES AND THE LIQUID MEDIUM IS CAUSED TO FLOW OUTWARDLY FROM THE COLUMN AT A SCREENED OPENING POSITIONED INTERMEDIATE ITS ENDS. A WASH LIQUID IS INTRODUCED AT ONE END OF THE COLUMN TO DISPLACE THE LIQUID MEDIUM FROM THE INTERSTICES BETWEEN THE   PARTICLES AS THE POROUS BED MOVES THROUGH THE COLUMN. CONTROL MEANS ARE PROVIDED TO CONTROL THE RELATIONSHIP AMONG THE PRESSURES AT THE ENDS OF THE POROUS BED AND THE PRESSURE AT SUCH SCREENED OPENING SO AS TO MAXIMIZE THE RATE AT WHICH THE PARTICLES CAN BE REMOVED AT THE OUTPUT END OF THE COLUMN.

United States Patent [72] Inventors Ronald F. Probstein 53 Jordan Road,Brookline, Mass. 02146; Josef Shwartz, Cambridge, Mass. ((49 EinsteinSt., Haifa, Israel [21] Appl. No. 748,811 [22] Filed July 30, 1968 [45]Patented June 28, 1971 [54] METHOD OF SEPARATING SOLID PARTICLES FROM ASLURRY WITH WASH COLUMN SEPARATORS 6 Claims, 5 Drawing Figs.

[52] U.S.Cl 210/66, 62/58. 210/71, 210/433 [51] Int. Cl B0ld 37/04 [50]Field 01 Search 62/58; 210/66, 71

[56] References Cited UNITED STATES PATENTS 3,400,549 9/1968 Karnofsky62/58 mzssuas CONTROL Primary Examiner-Samih N. Zahama Attorney-Roberts,Cushman & Grover ABSTRACT: An apparatus and method for removing solidparticles, such as ice crystals, from a mixture of said particles in aliquid medium, such as brine, wherein a wash column separator is used toform a porous bed of said particles and the liquid medium is caused toflow outwardly from the column at a screened opening positionedintermediate its ends. A wash liquid is introduced at one end of thecolumn to displace the liquid medium from the interstices between theparticles as the porous bed moves through the column. Control means areprovided to control the relationship among the pressures at the ends ofthe porous bed and the pressure at such screened opening so as tomaximize the rate at which the particles can be removed at the outputend of the column.

WASH uoulo TANK 22 pnzssuns 25 OR vacuum 28 CONTROL /I9 b l uoumDISCHARGE 1 3| TANK PATENTEU JUNE-8 l9?! SHEET 1 BF 2 x242. Q50: Iwis6528 mmzwwmmm fi v.23. Exam INVENTORS RONALD F PROBSTEIN JOSEF SHWARTZBY THOMAS COOCH v MARTIN M SANTA ROBERT F O CONNELL ATTORNEYS PATENTEUJUN28 1971 SHEET 2 OF 2 POROUS BED LENGTH (L|+L2) WEZOEZMEEZOZHMBEZOFUDQOEQ PRESSURE DIFFERENCE (p -p ACROSS POROUS BED (NONDIMENSIONAL)FIG 3 IN COLUMN DIAMETERS d (NONDIhIgIEGNSIONAL) PRESSURE DI FF ER ENCE(p INVENTORS ACROSS POROUS BED (NONDIHENSIONA RONALD E PROBSTEIN JOSEFSHWARTZ BY TH OMAS COOCH MARTIN MSANTA ROBERT F. OCONNELL ATTORNEYSMETHOD OF SEPARATING SOLID PARTICLES FROM A SLURRY WITH WASH COLUMNSEPARATORS This invention relates generally to apparatus and methods forremoving solid particles from a mixture of said particles in a liquidmedium and, more particularly, to apparatus and methods for removing icecrystals from a saline solution.

One method that has been used for removing substantially fresh, orsalt-free, water from sea water, or other salt water solution, is afreeze-distillation process wherein a slurry of ice crystals mixed in abrine solution is produced by appropriately refrigerating salt water.The ice particles formed by freezing the salt water consist ofsubstantially pure water while the salt accumulates in the unsolidifiedliquid phase or brine. Before the ice crystals can be melted to obtainpure water they are separated from the brine solution which adheres tothe crystal surfaces because of interfacial tension. Since the icecrystals produced in the freeze-distillation process are relativelysmall, the total area of ice exposed to the brine is large and suchsurface tension forces assume major importance.

Conventional methods used up to now for the separation of ice crystalsfrom a brine solution have proved to ,be too inefficient for use inconverting sea water to fresh water. For example, attempts to overcomethe difficulties associated with the separation of ice crystals frombrine have involved the use of centrifuging, gravity drainage, vacuumfiltration, and compression processes which have been found to betechnically or economically unfeasible. One potentially effectiveseparation method which has been used, however, involves a displacementprocess in which the brine is displaced from the interstices of the icecrystals by a second liquid, such as fresh water, which has a surfacetension nearly equal to that of the brine. In a salt water conversionprocess the clean output product water can be utilized as the displacingliquid.

One particular continuously operated device based on the displacementprinciple has made use of a vertically oriented wash column separator inwhich a slurry of brine and ice crystals is introduced at orsubstantially near the bottom of such column. The ice crystalsconsolidate within the column so that a porous ice bed, or ice plug,which moves continuously upward is formed and a displacing liquid, orwash water, is added from above. In one particular embodiment usedpreviously, the brine itself is utilized to provide the driving forcefor moving the ice particles upwardly in the column and one or morescreened openings are located in the vertical side walls of the columnat a position intermediate the upper and lower sections thereof throughwhich screened openings the brine is discharged. The stream of brinethus carries the ice particles in an upward direction where theyultimately consolidate into a porous bed which moves continuously bymeans of a pressure'difference maintained across the bed. As the porousice bed moves upward through the wash water, the brine is displaced fromthe interstices of the porous bed and salt-free crystals are suitablyharvested, or removed, at the top of the column and transferred to anappropriate melting apparatus.

However, such present day wash column structures are not normallyemployed under optimum operating conditions so that maximization of theproduction rate, either from a technical or from an economic viewpoint,has not been achieved. Moreover, production rates have been found tovary for apparently identical sets of operating conditions. Thus, thereliability of present day wash column separators has been relativelypoor.

This invention on the other hand provides for a wash column separationsystem in which production rates at least an order of magnitude higherthan those presently availablefor use may be reliablyrealized. In suchsystem production rates are maximized by appropriately controlling therelationship among the pressures which exist at the bottom of the porousbed, at the top of the porous bed, and at the discharge screens. Thus,the pressure differentials between the top of the bed and the bottom ofthe bed, between the top of the bed and the discharge screen, andbetween the discharge screen and the bottom of the bed are subject tosuitable controls so that an optimum formation of the porous bed,particularly in terms of its dimension below the discharge screenedopenings is maintained to produce a maximum production rate.

Apparatus used previously has never made use of such a three-waypressure control system and in fact has avoided the use of such pressurecontrols in the belief that such pressures are inherently fixed by thewash column structure and that their values cannot be changed so as toimprove production rates. For example, prior to this invention, those inthe art have believed that the distance from the bottom of the porousbed to the discharge screen is not at all subject to control and thatsuch distance is unalterably fixed for a particular wash columnconfiguration and dimension. This invention, on the other hand, showsthat the pressure differential between the discharge screen and thebottom of the porous bed, for example, should be effectively controlledso as to minimize such distance and that such control substantiallyincreases the efficiency of operation of the overall system.

The detailed structure and method of operation of the invention isdescribed more clearly with reference to the attached drawings wherein:

FIG. 1 shows a block diagram of a particular embodiment of a wash columnseparator system in accordance with the invention;

FIG. 2 shows a graph depicting the relationship between the productionrate and the dimensions of the porous ice bed of the invention shown inFIG. 1;

FIG. 3 shows a graph depicting the relationship between the productionrate and the pressure across the porous bed for different dimensions ofthe bed in the invention of FIG. 1;

FIG. 4 shows a graph depicting the relationship between the backpressure at the screen required to maintain a nearly zero loss of washliquid and the pressure difference across the porous bed for differentdimensions of the bed in the invention of FIG. 1; and

FIG. 5 shows a portion of the wash column separator system of FIG. I.

In FIG. 1 there is shown a wash column 10 having an input end 11 intowhich a slurry 12 comprising a plurality of solid particles, such as icecrystals, in a liquid medium, such as brine, is introduced. The slurryis obtained from an appropriate slurry tank 13 through suitable piping14. A pressure control system 15 is utilized at the slurry tank tocontrol the pressure of the slurry as it enters the input end 11 ofcolumn 10 and, hence, to control the pressure p,, at the bottom of theporous bed which is subsequently formed within the column as discussedbelow. Column 10 may be ofa cylindrical configuration having a screeneddischarge opening 16 located at a position intermediate its upper andlower end sections. In the cylindrical configuration, such screenedopening is preferably in the form of a continuous screen about theentire periphery of the column. Alternatively, the column may have adifferent cross-sectional geometric configuration and, if rectangular,for example, preferably includes a pair of screens located in oppositewalls of the column with the intervening walls maintained in solid form.

A porous bed 17 of ice crystals is thereby ultimately formed in thecolumn 10 during the continuous operation of the column, such bed havinga lower end surface 18 and an upper end surface 19 as shown. A suitableinput pipe 20 is located at the upper end of column 10 for introducing awash liquid 21, such as fresh water, from a wash liquid tank 22 throughappropriate piping 23. The wash liquid thereupon forms a wash waterlayer 24 at the upper end of column 10 through which the upper end ofporous bed 17 moves in its vertical travel upward. The wash water layeris separated from the brine containing region 31 by the interface 32.The pressure of the wash liquid and, hence, the pressure p at the upperend of the porous bed 17 can be controlled by pressure control means 30.An appropriate removal means, such as a mechanical scraper 25 ofappropriate known configuration not shown in detail, is also located atthe upper end of column 10 and is used to remove continuously the upperportion of porous bed 17 as it moves upwardly. Such removed portion canbe appropriately conveyed to a melter (not shown) by suitable conveyingmeans (also not shown).

As the slurry moves upwardly, the liquid medium, or brine, in which theice crystals are carried into the column, is conveyed outwardly throughscreened opening 16 to a liquid discharge tank 26, again via appropriatepiping 27. The outward pressure p, at screened opening 16 is controlledby a suitable pressure or vacuum control system 28 operated inconjunction with liquid discharge tank 26. Consequently, the region ofcolumn at the screened opening is enclosed in a suitably sealedenclosure 29.

In discussing the operation of the structure described with reference toFIG. ii, the following dimensional and operating parameters of interestare defined. The pressure parameters p,, p,, and p, are defined as thepressures at the bottom surface of porous bed 17, at the upper surfaceof porous bed 17, and at the screened opening 16, respectively, as shownin the drawing. The distance L, extends from the center of screenedopening 16 to the bottom end 18 of porous bed I7, while the distance L,extends from the center of screened opening 16 to the upper end 19 ofsaid bed, as shown also in the drawing. The distance I: represents theheight of the screened opening 16, and d represents the diameter (orwidth)'of the column. Other factors, such as the permeability of theporous bed and the external restraining forces operating on the porousbed are discussed below as their pertinence becomes apparent in thedescription which follows.

In the operation of the wash column separator, the rate of removal ofice crystals from the top of the column by scraper means 25 is arrangedunder continuous operating conditions to equal the rate of arrival ofice crystals at the bottom of the porous bed so that the overall lengthof the porous bed remains substantially constant during operation. Thedescription which follows of the operation of the column shown in FIG. 1concerns the operating time period after the so-called startup periodduring which operating time period the ice crystals have consolidatedinto a porous bed which extends upward to the full length of the columnabove the screens (the distance L,) and to a certain distance below thescreens (the distance L,). Under such conditions, the production rate ofthe column is continuous and uniform in time.

In such continuous operation the slurry 12 is continuously introduced atthe bottom of the column at a constant rate and the liquid brine filtersupwardly through the bottom section of porous bed 17 and divergesoutwardly toward the screened openings 16. Wash water is supplied at thetop of the porous bed and forms a layer 24 through which the porous bedmoves so that any brine remaining within the interstices of the icecrystals is displaced by the wash water and is directed downwardlytoward the screened opening 16. It is desirable in the operation of thedevice that the pressure differential (p p be maintained at a value suchthat a minimum amount of wash water is dispersed or filtered belowinterface 32 and, hence, through the porous bed and out from thescreened opening 16. Thus, the value of such pressure differential isarranged so that the volume of layer 24 remains substantially constantand whatever small amount of wash liquid is lost through such filteringaction is replaced by a sufficient amount of liquid introduced from thewash liquid tank 22. The pressure differential (p,p,) is maintained at asufficiently large value to provide the driving force required to movethe porous bed 17 with its entrained fluids at essentially constantvelocity.

A sharp increase in the production rate, that is, the rate at which iceparticles can be removed from the upper portion of porous bed 17, occurswith a decrease in the length L, of porous ice bed 17 below the centerof screened opening 16. Since large volumes of brine have to bedischarged through the lower portion of the porous bed and, thence,through screened opening 16, the portion of the porous bed below thescreens presents a considerable resistance to the fluid flow, thus,severely limiting the rate of brine discharge, the rate of ice transportto the column, and the rise of ice in the column. A decrease in thelength L, of porous bed 17 below the screened opening increases both thebrine rate of flow and the porous bed upward velocity and,correspondingly, has been found to produce a remarkable increase in theproduction rate.

Since it is advantageous, therefore, to operate the column with as smalla value for L, as is possible, it appears, at least theoretically, thatan optimizing of the production rate may occur if L, is reduced to zero.However, there is a minimum practicable length which should be usedsince, if L, is reduced essentially to zero, the stream of mixed icecrystals and brine impinges directly on screened opening 16 and wouldtend to clog portions thereof and interfere with the continuousoperation of the column. Therefore, it is preferable to operate thecolumn so as to allow ice crystals to consolidate into a solid porousbed at some point just before reaching the screened opening which willavoid such interference with the continuous operation of the column. Thelocation of this point (i.e. the distance L,) can be controlled bycontrolling the relationship among the pressures p,, p, and p If L, isfixed at a minimum practicable value, the production rate can also befurther increased by increasing the length L, of the porous bed abovethe screened opening. An increase in L requires a proportional reductionin the pressure p at the screens to maintain the wash water velocity ata value approximately equal to zero. Reducing p will increase thepressure difference (p,p,,), and therefore also the rate of brinedischarge and the production rate of the column.

Furthermore, if the friction force between the porous bed and the columnwalls is a significant factor in the total mechanical force whichrestrains the motion of the bed, an increase in L requires aproportional increase in p, to keep the forces on the porous ice bedappropriately balanced so as to move the bed upwardly at a uniform rateunder continuous operation. Thus, increasing the pressure ,0, willfurther increase the rate of brine discharge and, consequently, theproduction rate of the wash column separator. The production rateappears to increase indefinitely with an increasing L and, hence, thelimitation on the length of L will in a practical sense be dictatedprimarily by the economic factors involved in the construction of acolumn having reasonable dimensions in view of the operating andconstruction costs desired.

The relationship of the production rate to the dimensions L, and L isgraphically illustrated in FIG. 2 wherein a decrease in L, for a fixed Land an increase in L for a fixed L,, are shown to produce remarkableincreases in production rate for particular fixed values of the overalllength (L,+L of porous bed 17. In accordance with standard engineeringpractice the production rates and porous bed lengths are plotted asnondimensional quantities as indicated therein.

The production rate of such a wash column separator is also essentiallylinearly proportional to the pressure difference (pl-p acting across theporous bed. For the continuous operation of the column the driving forceon t he porous bed 17, resulting from the pressure difference (p,p,),should be at least equal, however, to the mechanical restraining forcesacting on the bed. Such forces which, of course, are always present insome form may arise, for example, from the friction between the ice bedand the wall of the column, from the downward force exerted on theporous bed by the mechanical scraper, or from any other external forcewhich acts to restrain the movement of the ice bed, apart from thoseforces arising as a result of the specific controlled pressures p,, pand p, applied as previously discussed above. For particular values ofL, and L it has been found that such restraining force, and thecorresponding pressure difference (p,p,), should be increased as much aspossible in order to increase the rate of production ofthe column asshown graphically in FIG. 3. As in FIG. 2, the production rates andpressure differences are also plotted as nondimensional quantities.

While not absolutely necessary, it may be preferable to preset the totalrestraining force acting on the porous bed at a fixed value priortooperation of the column. This may be acticles in a liquid carrier.Variations in the structure and the complished, for example, byutilizing an appropriate mechanimethod of operation thereof may occur tothose skilled in the cal member such as a blade 33, shown in FIG. 5,mounted at art without departing from the scope of this invention.Hence, the upper end of the column in contact with the upper portion theinvention is not to be construed as limited to the particular of porousbed 17 to provide a suitable downward pressure at 5 embodiment as shownin the drawings and described above the top of the porous bed or byincreasing the frictional force except as defined by the appendedclaims.

between the bed and the walls by roughening or otherwise we Claim;

treating the wall surfaces. Moreover, it may be further desira- A methodfor F Solid Particles from a Slurry ble in some applications to providefor control of the value of taining Said Solid Panicles in a liquidmedium comprising the the restraining force acting on the porous bedduring operamalls I tion of the column. One such means for varying therestraining f fl f) f P end Sectiofl Ora column; force may utilize blade33, as mentioned above, which can be ls hqud f flow from sald column ata mounted so as to be capable of movement toward and away positionintermediate the input and output ends thereof from the column incontact with porous bed 17. The restraina Q P of Sam paft'cles Sublect aing force so acting uponthe porous bed, thus, may be varied as 5restrammg force w'thm m Porous bed desired during operation byappropriately moving such blade ends extendmg be 0nd mtermed'ate posmon;

introducing a wash iquid into said column to form a substantiallyconstant layer thereof at the output end section of said column, saidwash liquid thereby displacing said either manually or by suitable andwell-known mechanical control means (not shown), in the directions shownby arrow 34. Alternatively, the angle of the blade surface relative tothe 20 liquid medium from the interstices between said solid partopsurface of the porous bed may be varied either In con unctides in Saidporous bed as Said porous bed moves tion with the movement of the bladetoward and away from through Said layer; and

the porous bed or independently thereof so as to further vary suchrestraining force;

For every choice of the pressures p, and p and the lengths 5 L and L aparticular setting of the back pressure behind the screen, p,,, isrequired-so as to minimize the net downward controlling the relationshipamong the pressures at the input and output ends of said porous bed andthe pressure at said intermediate position so as to overcome saidrestraining force and to move said porous bed toward said output end ofsaid column at a substantially constant velocity 1055. Of washingliquid, as depicted in AS in and so as to maintain the distance from theinput end of FIGS. 2 and 3, the pressures involved here are also plottedas id porous b d 1 id i di i i at a nondimmsional quantities preselectedvalue such that said input end of said porous Th h g it Of t Screened pg 16 will have n y a bed is at apoint just below said intermediateposition.

moderate effect on the production rate of the wash column 2, A methodfor separating solid particles from a slurry separator. For very lowvalues of h, the values of p which are taining said solid particles in aliquid medium in accordance required to maintain a sufficiently highvalue of brine with claim I and further comprising the step of dischargerate at the screened opening may not be achievable removing said solidparticles as said porous bed moves to in practice. It is recommended,therefore, to use a screen the output end of said column.

height h which is an appreciable fraction, for example, at least 3. Amethod for separating solid particles from a slurry conapproximately 20percent, of the column diameter d. taining said solid particles in aliquid medium in accordance In summary, a preferred structure and methodof operation with claim 2 and further comprising the step of of the washcolumn separator, in accordance with the present controlling the valueof said restraining force acting on said invention, will include aporousbed with a short lower leg L porous bed during operation of said column.

and a long upper leg L a screen height which is an apprecia- 4. A methodof separating solid particles from a slurry conble fraction of thecolumn diameter, a pressure difference taining said solid particles in aliquid medium in accordance (p -0J2) which is sufficient to drive theporous bed at a conwith claim 1 wherein Said preselected value of thedistance stant rate against its mechanical restraining forces and asuitafrom the i put nd f a d p rous bed to said intermediate blepressure setting p behind the screen which will minimize position ismade as small as possible without interfering with the loss of washliquid through the screen. The setting of all of the flow ofliquid atSaid t rmediate position. the above mentioned parameters can beoptimized so as to ob- A method f pa ing li particles from a slurryconmi h maximum production rate per unit cost f comm taming said solidparticles in a liquid medium in accordance i l d opgrariom with claim 4and further comprising the step of further con- Broadly stated, theoverall wash column separator structure f the relatlonshlp among sflid PFP 50 as to maintain is constructed so that appropriate control meansare provided Q' from Sam lmermedlae P0Smon the P end to control therelationships among the pressures p,, p and p 0f Porous bed at a P etoproduce a maximum rate of removal of ice particles at the A mefhod f jsollfi R m a Slurry output end fth column taming said solid particles Ina liqu d medium n accordance While the structure has been describedparticularly as clam 1 wherein the rekftlonshll among said P F Fproviding a means f removing ice particles from an ice furthercontrolled so as to ad ust the loss of said wash liquid at crystal-brinemixture, it is clear that such structure can be 'utilmte'med'ate P izedto separate any solid particles from a mixture of such par-

